The goal of the proposed research is to examine in detail the input-output organization of the layers of the superior colliculus below the stratum opticum that provide sensorimotor mechanisms for orientation responses. We will determine the relationships to one another and to tectal efferent neurons of several major collicular afferent conections, most of which terminate in a discontinuous "columnar" pattern. Our efforts will be concentrated especially on inputs to the colliculus from the frontal and parietal cerebral cortex, the substantia nigra, the trigeminal complex, the cerebellum, and the retina. The studies are anatomical using several contemporary axoplasmic transport technics alone and in combination at the light microscopic level of analysis. Completion of these projects will provide a clearer understanding of how the superior colliculus carries out its important role in orientation behavior, and a groundwork for meaningful physiological and behavioral investigations and should, more generally, yield new insight into the modular organization of the central nervous system of higher mammals.